1. Field of the Invention
The present invention relates to a digital time division mobile communication system having separate time slots for transmission and reception at a mobile station, and more particularly to an adjacent cell monitoring method.
2. Description of the Related Art
As described for example in Japanese Patent Laid-open No. 327139/89, this type of adjacent cell monitoring is performed with the object of realizing a mobile communication method that enables constant optimum connection with a base station without deterioration in transmission quality as a mobile station moves from one zone to another.
FIG. 1 is a block diagram showing the channel switching method in mobile communication of the prior art. Mobile communication exchange 61 is connected to public switch telephone net work 60, and base stations 62, 63, and 64 each having radio zones 65, 66, and 67, respectively, are each connected to mobile communication exchange 61.
While communicating with base station 63, mobile station 68 sequentially scans the control channel signals of base station 63 with which it is currently communicating as well as of adjacent base stations 62 and 64 at times other than during the time slots assigned to mobile station 68, receives signals, measures the reception quality of these signals, selects the optimum base station based on these measurement results, and transmits optimum base station information to base station 63 with which it is communicating. Based on this optimum base station information that it receives by way of base station 63, mobile communication exchange 61 then switches the base station connected to mobile station 68.
The above-described prior art has a disadvantage that a certain amount of time is required before the optimum base station is selected.
This delay arises from the necessity of measuring the reception quality of adjacent base stations before the optimum base station can be selected. Reception quality is found from the base station having the highest received signal strength after measuring the received signal strength of adjacent base stations. In order to eliminate the influence of fading, the measurement of received signal strength generally requires taking a plurality of measurements, for example, five measurements, spaced out by measurement intervals and calculating the average value. The number of received signal strength measurements required is therefore the product of the number of base stations in the vicinity and the number of received signal strength measurements per base station. Carrying out received signal strength measurements equal in number to this product in the time intervals outside the time slots assigned to the mobile station is time consuming.